I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved method and apparatus for determining the transmission data rates in a high speed wireless communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA/IS-95 Subscriber station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
In this specification, base station refers to the hardware with which the subscriber stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell, the teachings described in terms of cells are readily extended to sectors.
In the CDMA system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second subscriber station. The forward link refers to transmission from the base station to a subscriber station and the reverse link refers to transmission from the subscriber station to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
The subscriber station communicates with at least one base station during a communication. CDMA subscriber stations are capable of communicating with multiple base stations simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new base station before breaking the link with the previous base station. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled xe2x80x9cMOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Softer handoff is the process whereby the communication occurs over multiple sectors which are serviced by the same base station. The process of softer handoff is described in detail in copending U.S. patent application Ser. No. 08/763,498, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATIONxe2x80x9d, filed Dec. 11, 1996, now U.S. Pat. No. 5,933,787, issued Aug. 3, 1999, by Klein S. Gilheousen, assigned to the assignee of the present invention and incorporated by reference herein.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec wide with data rates as high as 14.4 Kbps.
A system completely dedicated to high speed wireless communications is disclosed in copending U.S. patent application Ser. No. 08/963,386 (the ""386 application), filed Nov. 3, 1997, entitled, xe2x80x9cMETHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSIONxe2x80x9d, which is assigned to the assignee of the present invention and incorporated by reference herein. In the ""386 application, the base station transmits to subscriber stations by sending frames that include a pilot burst time multiplexed in to the frame and transmitted at a rate based on channel information transmitted from the subscriber station to the base station.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques which require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 08/743,688, entitled xe2x80x9cSOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDSxe2x80x9d, filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462, issued Aug. 3, 1999, by N.T. Sindhushayana, et al., assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common quality of service (QOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the QOS can be different from user to user, can be negotiated, and should be subject to some fairness constraints. The QOS that a data communication system provides to a subscriber is typically described by the delay, average throughput, blockage probability, connection loss probability experienced during service time.
A wireless data communication system can typically provide a range of transmission data rates both in the forward and reverse links. These transmission data rates are allocated to the various active traffic sources according to a strategy, identified as medium access control that must account for the fact that the sources typically offer different incoming information data rates, depending essentially on the selected data application. Also, channel conditions and overall system load should be considered when allocating transmission data rate to a specific subscriber.
Medium access control amounts to allocating the resource to the active subscriber stations in the network in a way that optimizes the trade-off between overall system throughput, QOS, and algorithm complexity. While in the forward link one can exploit the xe2x80x9cone-to-manyxe2x80x9d nature of the transmission to perform optimal centralized resource allocation at the base station, in the xe2x80x9cmany-to-onexe2x80x9d reverse link the problem of optimization of the medium access control strategy is complex, and can be solved with a centralized approach at the base station, or with a distributed approach at the subscriber stations. Although many of the techniques described herein may be extended to the medium access control of the forward link signals, the focus of the present invention is set on medium access control for the reverse link.
The information that should be used to perform resource allocation in the reverse link resides both at the network of base stations and at the subscriber stations. Specifically, at the network side resides the information pertaining to the instantaneous traffic load and spare capacity of each base station. The load can be quantified for example by the rise of the overall received energy over the floor set by the noise power spectral density. The spare capacity is the difference between the maximum allowable load that prevents network instability and the instantaneous load. At the subscriber station resides information about terminal class (for example maximum transmission power, transmission buffer size, supported data rate set), channel conditions (for example signal-to-noise plus interference ratio for all received pilots, transmit power headroom), and traffic source state (for example buffer state, buffer overflow, average throughput in the past, delay statistics). In principle, information can be exchanged between the network and the subscribers, but this involves signaling over the air interface which implies a waste of resources and a delay in the decision making process.
A first problem is therefore to design a medium access control strategy for the reverse link that exploits in an optimal way the available information minimizing signaling messages. Also, it is desirable for the medium access control strategy to be robust in terms of changes in the subscriber station class and in the network topology. Another fundamental problem is resource allocation for a subscriber station in soft handoff. In this case the traffic load and spare capacity of all base stations involved in the soft-handoff (identified as base stations in the active set) must be considered, again possibly minimizing signaling in the network. Yet another fundamental problem is protection of base stations that are not in soft handoff with a particular subscriber station, but that nevertheless are connected to that subscriber station through an electromagnetic link with path loss comparable to those measured in the active set. These base stations are referred to herein as the candidate set.
The present invention, described in the following, is an efficient and novel method and apparatus designed to address and solve all the above mentioned fundamental problems for a reverse link medium access control strategy.
The present invention is a novel and improved method and apparatus for performing transmission data rate allocation in the reverse link of a high speed wireless communications network. The present invention forms a macro control loop with the network of base stations on one side and all the subscriber stations on the other side. Each subscriber station selects a data rate based on the amount of data queued for transmission. Adjusts this rate based on the available power headroom. This adjusted transmission rate is then adjusted again to account for protection of base stations in the candidate set of the subscriber station. This rate is then adjusted in accordance with signals indicative of the loading conditions of active set base stations of the subscriber station. The base stations react to the subscriber stations action by measuring their instantaneous traffic load and providing feedback in the form of soft busy tones. The method is referred to herein as Closed Loop Resource Allocation.
It is an objective of the present invention to optimize reverse link medium access control by placing the data rate allocation under the control of the subscriber station which has a greater amount of information by which to determine the transmission rate than do the elements on the network side. The subscriber has information regarding the amount of information it has queued to transmit, and the amount of available transmit power headroom, the signal-to-noise plus interference ratios in both the active set and the candidate set links, all of which are essential factors in selecting a reverse link transmission rate. The base stations do not have this information absent a significant amount of signaling, which is undesirable.
It is another objective of the present invention to prevent a subscriber station from creating unacceptable interference to candidate base stations by its reverse link transmission, thus enforcing candidate set protection.
It is another objective of the present invention to allow data rate allocation on a per-packet basis, to provide the flexibility that is necessary to provide efficient service to subscriber stations offering traffic with high burstiness.
It is another objective of the present invention to provide fairness in resource allocation among the subscriber stations by taking into account the average throughput in the recent past and the possible buffer overflow condition.
It is another objective of this invention to provide efficient reverse link medium access control without requiring any signaling in the backhaul, between base station transceivers and base station controllers, even when the subscriber station is in soft handoff. This is highly desirable because it makes resource allocation independent from the network architecture and the associated transmission and processing delays.
It is another objective of the present invention to minimize the necessary signaling on the air interface.
It is another objective of the present invention to avoid resource waste that occurs when the rate used by the subscriber station is smaller than the allocated rate. In fact, in closed loop resource allocation the allocated rate and the used rate are always coincident.
It is yet another objective of the present invention to provide soft multi-bit busy tones that indicate not only whether a base station is in an overload condition or not, but also provides some indication of the extent of its loading.